In order to facilitate fitting on the engine, so-called “elastic” poly-V belts have recently been introduced on the automobile market.
“Elastic” or “elastically extensible” belts, that is having resistant inserts or cords with a relatively low tensile modulus of elasticity and able to allow an elastic elongation of the belt during assembly, have been known for decades in the field of industrial applications, for example in electric household appliances, in which the loads transmitted and the requirements of duration are not as severe as in the automobile field; these belts are usually fitted manually since their tensile modulus of elasticity is very low and they can be stretched elastically with acceptable stresses.
For automobile applications, in which there is the similar problem of fitting the belt onto the transmission pulleys without varying the centre distance between the pulleys but rather by extending the belt, elastic belts have only recently been introduced on the market. In fact, only materials of the latest generation allow acceptable compromises to be found between the need to “elongate” the belt elastically and the strict functional requirements of automobile applications.
A problem connected with the known elastic belts is how to maintain over time a functionally acceptable level of tension for the transmission. The tension decay over time is normally related to the tensile modulus of elasticity of the belt, in the sense that, when the modulus is reduced there is a corresponding increase in the tension decay.
In practice, this means that when the modulus of an elastic belt is reduced beyond certain limits, the belt tends to lose tension rapidly while working and therefore works slack over time until its ability to transmit the load is compromised.
It is known how a reduction of the modulus of a belt may be obtained by increasing the number of twists of the cords, while the materials and all other constructive characteristics remain the same.
In particular, cords are known that are made of yarn that has undergone a first twisting process to obtain plied yarns, which are then subjected to a second twisting process in the opposite direction. It is known that increasing the number of twists per unit length of the yarns and of the plied yarns determines a reduction in the tensile modulus of elasticity of the cord.
However, cords with a high number of twists per unit length present a greater tendency to elastic decay.
On the basis of the above, the need to reduce the tensile modulus of elasticity of a belt clashes with the antithetic need of ensuring that tension losses are acceptable for the application.